One of the shops I go to they have to input the length of the arrow into the machine, so it must make a difference to some extent.
Must be only one detector that senses both the leading and trailing edge of the object, then given the length of the projectile the speed can be calculated. I never saw one like that. I suspect it to be inherently inaccurate when compared to 2 or more detectors. Seems like differences in leading andtrailing edge geometrywould be a real problem. With 2 or more windows/detectors the length of the object is irrelevant and the shadow detected to both start and stop the timer are identical.